24 



A Synopsis of the p-incipal 



Satel- 

 lite. 



I. 

 II. 

 III. 

 IV. 



Sidereal Revolution. 



1<'1&1>27'33'',5 



3 ]3 13 42 ,0 



7 3 42 33 ,4 



16 16 31 49 ,7 



1'^769137788148 



3 55"il81017849 



7 154552783970 



16 6887697070B4 



Mean 

 Distance. 



5-812964 



9248679 



14-752401 



25-946860 



Mass. 



-00001 732&L 



00O0232355 

 0000884972 

 0000426591 



First SateUite. The inclination of the orbit of this sa- 

 tellite does not differ much from the plane of Jupiter's orbit. 

 Its eccentricity is insensible. 



Second Satellite. The eccentricity of the orbit of this 

 satellite is also insensible. The inclination of its orbit, to 

 that of its primary, is variable; as well as the position of 

 its nodes. 



Third Satellite. This satellite has a little eccentricity; 

 and the line of its apsides has a direct but variable motion : 

 the eccentricity itself is also subject to very sensible varia- 

 tions. The inclination of its orbit to that of Jupiter, and 

 the position of its nodes, are far from being uniform. 



Fourth Satellite. The eccentricity of this satellite is 

 greater than that of any of the other three ; and the line of 

 the apsides has an annual and direct motion of 42' 58", 7. 

 The inclination of its orbit, with the plane of Jupiter's or- 

 bit, forms an angle of about 2° 23' 48": but this angle, al- 

 though stationary about the middle of the last century, has 

 lately begun to increase very sensibly. At the same time 

 the motion of its nodes has begun to diminish. 



The motions of the first three satellites are related to each 

 other by a most singular analogy. For, the mean sidereal 

 or synodical motion of the first, added to twice that of the 

 third, is constantly equal to three times the mean motion 

 of the second. And, the mean sidereal or synodical longi- 

 tude of the first, minus three times that of the second, plui? 

 twice that of the third, is always equal to two right angles. 



The satellites of Jupiter are liable to be eclipsed by pass- 

 ing through his shadow; and, on the other hand, they are 

 frequently seen to pass over his disk, and eclipse a portion 

 of his surface. This happens, to the first and second sa- 

 tellite, at every revolutioti : the third very rarely escapes in 

 each revolution : but, the fourth (on account of its gfeac 

 distance and inclination) is seldom obscured. 



These eclipses are of great utility in enabling us to deter- 

 mine the longitude of places, by their observation : and 

 they likewise exhibit some curious phaenomena with respect 

 to light. 



From 



